The cluster will form as the inside temperature of the hive falls. Lots of brood and a lot of bees in a well insulated hive may mean the colony does not start to cluster as soon as a small colony in an over-sized or colder hive.
All sorts of considerations, the above and day-time temperatures, 'vain' space in the hive,etc, but it basically comes down to internal hive temperature; the bees know when they need to huddle together to keep the broodnest (reduced) and winter bees warm enough. Any bees experiencing temperatures below about eight degrees Celsius for extended periods will be brown bread.
Two years ago and there was little clustering before December and bees were still actively flying and foraging right up to Christmas Eve, although they may have been in a huddle from late November, onwards, for a few days at a time. Last year, they were all totally clustered by Christmas - and had been for a month and probably more. This year? Who knows yet. We will find out in our particular locale when it happens. Simple as that; the bees will decide; they know best.
With regards brooding. Two years ago - lots of brood into December, last year brood was very much reduced early November and this year I have not checked but expect eggs and young brood are severely diminished in number even now.
Restarting brooding. Two years ago, soon after the middle of February; last year more like the middle of March!
Sooo, it all depends on the weather and forage. Whatever you do will make little difference if the colonies are strong, healthy and dry - along with adequate but not excesssive ventilation. The only other thing the beekeeeper can be helping them with is that of a sufficient supply of heating fuel for the winter months, especially after snitching all their hard-earned stores earlier in the year.
Regards, RAB